Drill bit with improved core destroying means



May 28, 1963 o. HAMMER 3,091,300

DRILL BIT WITH IMPROVED CORE DESTROYING MEANS Filed 001;. 5, 1960 3Sheets-Sheet 1 0/20 Ham? er INVENTOR.

ATTORNEY) May 28, 1963 o. HAMMER 3,091,300

DRILL BIT WITH IMPROVED CORE DESTROYING MEANS Filed Oct. 5, 1960 5Sheets-Sheet 2 0/20 Ham/77 er INVENTOR.

ATTOR/VfKJ y 1963 o. HAMMER 3,091,300

DRILL BIT WITH IMPROVED CORE DESTROYING MEANS Filed Oct. 5. 1960 3SheetsSheet 3 0/20 Ham/77 er INVENTOR.

ATTO/PA/EYJ United States Patent C) 3,091,300 DRILL BIT WITH IMPROVEDCORE DESTROYING MEANS Otto Hammer, Dallas, Tex., assignor to DresserIndustries, Inc., Dallas, Tcx., a corporation of Delaware Filed Get. 5,1960, Ser. No. 60,688 5 Claims. (Cl. 175-333) The present inventionrelates to drill bits and more particularly has reference to that typeof drill bit adapted to drill in earth formations by rotary motion ofthe bit,- and to a rotary cutter adapted to form a part of such bit.

It is well known that in the drilling of holes in earth formations bythe rotary drilling method, one of the more difficult portions of thehole to drill is that portion substantially at the center of the holeand closely adjacent thereto, because at such locations there is littletransverse motion of the cutters of the bit with respect to theformation when such cutters are formed and mounted on the bits in theusual fashion. One of the most common types of drill bits employed inthe rotary drilling of wells in earth formations is the type employingone or more cutters which are generally conical in outer contour andwhich are ordinarily mounted with their bases radially outermost withrespect to the axis of rotation of the drill bit and with their noseends or smaller ends closest to the center of the bore hole. Suchcutters are also customarily mounted on axes which extend downwardly andinwardly with respect to the bit axis so that the conical sides of thecutters will roll on the bottom of the hole against the formation with amore or less true rolling action. The outer or gauge forming surface ofthe cone bearing against the formation produces a thrust on the cone ina radially inward direction. This produces a bearing problem requiring abearing capable of taking this thrust. It has been customary, in orderto provide for drilling of the formation at and near the center of thehole, to provide for one of the cutters to have an elongated noseportion extending to or even past the center of the hole. However, suchelongated nose portion is necessarily of very small cross section in aplane transverse to its axis and can only be provided with a few teeth.The result is that it is diflicult to make such a nose portion strongenough so that it will not be damaged during the drilling operation anddiflicult to provide bearing capacity for the cutter having such anelongated nose portion, such that the drilling forces exerted on thenose portion will be adequately supported. Neither does such a noseportion provide any outward radial thrust which would assist inovercoming the inward radial thrust from the gauge surface action.Finally, it is in many cases impossible to provide sufiicient teeth onsuch a nose portion so that an adequate number of contacts with thecentral portion of the hole will be made to provide a proper cuttingaction.

It is, therefore, one of the objects of this invention to provide adrill bit in which the central portion of the hole may be subjected toadequate cutting action without necessity for an elongated nose on anycutter.

Another object is to provide a drill bit which will provide adequatecutting action adjacent the center of the bottom of a hole Withoutnecessity for a cutter portion of such small cross section that it willbe of doubtful strength.

Another object of this invention is to provide a cutter of the characterdescribed which will provide a greater number of cutter teeth engagingthe central portion of the bottom of a hole with a cutting action thanhas been possible with designs heretofore employed.

Another object of this invention is to provide a drill bit of thecharacter described in which all of the cutters of a multi-cone drillbit have teeth thereon adapted to "ice engage the formation adjacent thecenter of the bottom of a hole being drilled.

Another object is to provide a drill bit of the character described inwhich adequate bearings can readily be provided for the portions of thecutters adapted to drill the formation adjacent the center of the bottomof a hole.

Another object is to provide a bit with cones whose action on the centerportion of the formation will partially or entirely counteract theradially inward thrust resulting from the action of the gauge formingsurfaces of the cones.

Other objects and advantages of this invention will be come apparentfrom the following description taken in connection with the accompanyingdrawings wherein are set forth by way of illustration and examplecertain embodiments of this invention.

In the drawings:

FIG. 1 is a fragmentary portion of a drill bit constructed in accordancewith this invention, a portion of the body thereof being broken away andshown in longi--.

tudinal cross section to illustrate the interior thereof, and the singlecutter included in this figure being shown in longitudinal crosssection, the cutter being illustrated in operative position with respectto the formation at the bottom of a hole being drilled so as toillustrate the drilling action of the cutter adjacent the center of thebottom of such hole;

FIG. 2 is a diagrammatic bottom plan view of a threecone cutter bitconstructed in accordance with this invention, the shapes of the noseportions of the cutters and of the cutter teeth thereon adapted to cutthe formation adjacent the center of the bottom of a hole being drilledbeing shown merely by lines indicating the cutting crests of such teeth;and the exemplary disposition of rows of teeth for cutting the outerportions of the bottom of the hole being shown only by lines outliningsuch rows;

FIGS. 3, 4 and 5 are diagrammatic fragmentary views in Vertical crosssection through the central portion of the bot-tom of a hole beingdrilled and showing respectively the action on the formation of the noseportions of the cutters of a bit constructed in accordance with thisinvention;

FIGS. 3A, 4A and 5A are enlarged scale end elevations of the noseportions only of the respective cutters shown in FIG. 2, the portions ofthe cutters other than the noses or inner end portions being brokenaway;

FIG. 6 is a view similar to FIGS. 3A, 4A, and 5A but illustrating .amodified arrangement of cutter teeth having elongated crests on theinner end of a cutter, skewed so as to lie at an inclination to a radialline therethroug-h.

FIGS. 7, 8 and 9 are mutually similar fragmentary views each showing insection along a plane through the axes of two adjacent cutters, said twoof the cutters of a three-cone bit and illustrating the manner in whichthe teeth thereof .interfit in such plane, these cutters being providedwith teeth of the so-called insert or compact yr FIGS. 7A, 8A and 9A areviews similar to FIGS. 3A 4A and 5A but illustrating the nose portionsof the respective cutters in FIGS. 7, 8 and 9.

The term teeth is intended to include cutting elements formed of hardmetal compacts, diamonds, or the like.

This invention, by providing rotating cutters with blunt ends havingteeth directed axially of the cutter adjacent the rotational axis of thebit, and by arranging those ends to rotate in engagement with and cutaway and form a conical upper end surface on the core which without suchcutting would be left at the center of the hole, provides for cuttingsuch core with a shearing, scraping and chipping action of aconsiderable number of teeth, instead of by a pivoting action of onlyone or two teeth. Also, by providing such cutters with such blunt endsof 3. different diameters although on cutters of identical lengths, onecutter nose may be made large enough to cut substantially the entire topof the core while the others cut portions of the top of the core wherethe cutting action of'the larger cutter nose is at least efficient.Furthermore, the reaction forces from engagementcf the blunt noses of-all the cones'consistof thrusts radially outwardly from the bit axisand thus tend to balance the inward thrust from'the' engagement of thebase end gauge cutting surfacesjofthe cones with the oute'r wall. of thehole.

Referring now in more detail to the drawings, the numeral '1 designatesa bit body which may be of any conventional construction having athreaded pin 2 on its upper end provided with external threads 3 for thepurpose of connecting it to the lower end of. a drillster'n. This upperend portion of the bit body is hollowed out in conventional fashion asillustratedlat l so as to provide for the fiow of drilling fluidfrom thedrill stem through the bit body and out through the fluid flow openings5. It is to be understood that this upper portion of the bit body isconventional in nature and that various types of bit body may beemployed in a bit constructed in ac.- cordance with thisinvention.

' Likewise, in conventional fashion the bit body 1 is provided with anydesired number of downwardly extendingj'cutter supporting legs 6 atintervals around its outer circumference; and each of these legs at itslower end has a downwardly and inwardly extending cutter bearing shaft 7providing races for suitable bearings such as the roller bearings 8adjacent the outer end of the shaft, the ball bearings 9 intermediatethe ends of the shaft, and a friction bearing 10 on the nose of theshaft as well as provision for :a thrust bearing surface 11 on the endof the shaft. The employment of roller and ball bearings in thepositions illustrated is conventional and it will be understoodthat'other types ofbearings may be employed in place of the frictionradial bearing 10 and thrust bearing 11' if found more desirable for anyparticular application of the bit.

It will be understood that FIG. 1 illustrates only one cutter of whatmay be, and is here disclosed as, a plural cutter bit, conventionalexcept as to the nose portions of the cutter or cutters. The most commonform is the tricone bit more diagrammatically illustrated in bot-tomplan view in FIG. 2. It is to be considered that FIG. 1 has othercutters of which the cutter 12 shown in FIG. 1 is exemplary in generalbut from which they will differ in certain features as will be presentlydescribed.

The cutter 12 is adapted to be locked in, place on the cutter bearingshaft 7 by means of the ball bearings 9 in a well-known conventionalmanner and may be provided with a sufiiciently large counterboreadjacent its nose to receive a hard bearing insert 13 adapted to engagethe radial friction bearing surf-ace 10 onthe shaft. Likewise, it may beprovided with a recess in the extreme nose portion for the purpose ofreceiving a thrust bearing insert 14 adapted to engage the frictionalbearing surface =11 on the nose of the shaft 7.

The teeth along the generally conical sides of the cutter 12 may beconventionally arranged but, in accordance with this invention, the axisof this cutter and other cutters of the same bit terminate short of thebit axis. This cutter has on its inner end a series of teeth -16 whosecrests, when the cutter rotates, will generate a surface which is shownas extending substantially at right angles to the cutter axis. Thissurface does not have to be at right angles to a cutter axis as it mayvary in shape from a flat surface, but should be a surface of rotationabout the cutter taxis and have its elements extending at a greaterangle to the cutter =axis than the cutter axis does with the bit axis sothat in operation that portion of the surface 15 which lies above thecutter :axis will extend upwardly and toward the bit axis.

This surface 15 should extend toward the cutter axis at an angle greaterthan that at which the normally conical side surfaces of the cutterextend toward the cutter axis, so that the cutter will have a blunt noseon which the cutter teeth 16 are formed for the purpose of cutting theformation adjacent the center of the bottom of the hole as will now bedescribed.

The arrangement of teeth on the sides of the cone 12 and of the othercones ofv the same bit'are substantially the same as in a conventionalbit and cut an annulus on the bottom of the bore hole, this annuluscovering all exceptv a small circular area in the center of the hole. Ifthis central area were not cut, then as the drilling on the annularareaproceeded, there would be formed in'the center of the hole a columnof uncut material commonly called a core. The conventional manner ofdisposing of this core and preventing it from building up so as tointerfere with and. butt againstthe ends of the cutters as the drillingproceeds is toprovide one of the cutters with an elongated nose portionthat extends substantially to or in some instances past the axis of thebore hole, and which has teeth extending in all radial directions fromthe nose portion of such cutter to cut away the core.

In accordance with this invention, such an elongated nose portion isdoneaway with and instead each of the cutters is provided with a blunt endon which are formed the end teeth or core cutting teeth such as 16extending and having cutting portions presented in a substantially axialdirection with respect to the axis of. the cutter. The core'cuttingteeth present axially directed cutting extremities whose aggregate areaon a cutter is less than the aggregate area by which said extremitiesare separated. This is clearly shown in FIGS. 3A, 4A, 5A, 6, 7A, 8A and9A. The effect is that of a seriesof cutting teeth separated by spacesproviding relief rather than an'interrupted wear surf-ace. When theseend teeth come in contact with the core left by the ordinary cuttingteeth on the sides of the cutters, the rolling actiontof the cone on thebottom of the borehole causes these end teeth to shave or shear acrossthe sloping top and sides of the short conical core along an inclineextending upwardly and inwardly toward the center of the bore hole so asto eifect its complete destruction. One cutter should have end teethwhich collectively extend far enough from the cutter axis tosubstantially destroy the core. Preferably the end teeth on all cuttersof a bit should generate and cut on the same conical or similar surfaceon the core.. Therefore, 'the cutting portions of correspondinglypositioned end teeth on every cutter of a bit should be the same. dis:tance from the center of rotation of the bit measured along the axis ofthe cutter in each case. This will prevent one or more cutters fromabsorbing all the cutting loadcn the core. Also, since the end teeth ofa single cutter cut only a narrow path in a vertical plane along oneside of the core at anyinstant, the endteeth of each cone should bearranged so that adiametrical path of substantial width cannot belocated across any cutter nose without encountering one of said teeth.This also is apparentin all of FIGS. 3A, 4A, 5A, 6, 7A, 8A and 9A.

However, with the end teeth so located, itis not pos: sible, in a bitwith more than two cutters, to have the end teeth of two adjacentcutters both 'of such radial extent relative to the cutter axis thatthey will cut substantially to the center of the core, because theywould interfere with, each other. In accordance with this invention, theend teeth of one cutter are made of an extent to cut substantially tothe center of the core while those of the adjacent cutters are made ofsufiiciently lesser extent to avoid interference with'such one cutter oreach other. Furthermore, the cutters .With end teeth of such lesserextent will haye such teeth of different radial extent from each otherto avoid interference between them. The result is as shown in FIG. 2with the largest nose, cutter and teeth cutting the entire surface ofthe core, the next larger cuttingonly a portion of the core over which,the teeth of the first-mentioned cutter cut, and the third cutting onlya portion of the core over which the teeth of the other two cutters cut.

When these end teeth are arranged as illustrated in FIGS. 1 to 5,inclusive, and have crests which are elongated and extend in radialdirections with respect to the rotational axis of the cutter, they willmove laterally so that they have a cutting action on the slopingsurfaces of the core and will continually shave off the material fromsuch sloping surfaces. However, this lateral movement is modified by thecontinuous rotation of the cutter about its axis and the axis of the bitso that instead of a simple scraping action, there is a compound actionof scraping, shearing and chipping which is constantly changing innature with respect to any given end tooth, depending on its rotationalposition on the cutter axis.

Referring more in detail to FIGS. 2, 3, 4, 5, 3A, 4A and 5A which areillustrative of some of the various actual tooth forms which may beemployed on the inner ends of the cutters, of which FIG. 1 shows apartial cross section, there is shown a three-cone bit having cutters12, 24 and 25. The series of core cutting end teeth 16 on cutter 12 haveelongated crests which are arranged in radial directions with respect tothe axis of the cutter 12. In the center of the blunt end 26 of thiscutter is a single end tooth having an elongated crest as shown at 28,the crest being disposed substantially along a diameter at the end 26.This single end tooth will cut the center of the core slope and theteeth 16 and 28 together will cut a path on each side of the center ofthe core slope.

The cutter 24 is provided with teeth 30 which will generate a largerdiameter surface 29 as compared with the surface 15 generated by thenose teeth on the cutter 12, but like the cutter 12 this cutter 24 hasthe end teeth 30 with elongated crests, each being arrangedsubstantially in a radial direction and spaced about the outer Zone ofthe blunt end 31. In the central portion of this blunt end, more than asingle tooth is required because of the larger area of this end ascompared with the end 26. Hence, four such elongated teeth areillustrated, these likewise being arranged with their crests extendingradially and their inner ends disposed closely adjacent the axis of thecutter 24. These four teeth cut the uncut formation remaining betweenthe teeth 16 and 28 of cutter 12, and with the other teeth 30 will cut apath wider than but centered along the same center of the core shape asthe path cut by the cutter 12.

The cutter 25 is similar to the other two but has a still larger bluntinner end 32 with end teeth adapted to cut a substantial distance alongthe sloping sides of the core so as to in effect complete itsdestruction. These end teeth have elongated crests extending radially asshown at 33, with these teeth disposed adjacent the outer periphery ofthe blunt end 32. In addition, closer to the center of the blunt end 32there are four similar end teeth likewise radially arranged. These teethwill generate the surface 34, and the innermost thereof will cut theformation left uncut between the inner and outer rows of teeth 30 oncutter 24.

It will be apparent that in a bit with the cutters having blunt endswith teeth thereon as illustrated in FIGS. 2, 3, 4, 5, 3A, 4A and 5A,the core will be disposed of as the bit advances by the end teeth on theblunt ends of all three cutters cutting along a single conical orinclined surface. Cutter 25, having the largest blunt end will cut overthe entire conical surface in the center of the hole. Cutter 24 will cuta narrower annular path spaced from the center and bottom of suchconical surface. Cutter 12 will cut a still narrower annular pathintermediate the limits of the path cut by cutter 24. The surfacesgenerated by all three sets of nose or end teeth are the same distancefrom the central axis of the bit measured along the respective axes ofthe cutters, so that all cutters will be required to perform work on theconical core at the same time. Thus, substantially 6 all of the teeth onthe blunt ends of these three cutters will move around the core intaking the cut in such a fashion that each tooth will generate theconical surface on the core and move along the core in a directiontransverse to the crest of the tooth, but with the compound action abovedescribed.

It would be preferable from the standpoint of cutting the core, that allthe blunt ends be large. However, to avoid interference with each otherand cut close to or entirely the center of the core, blunt ends ofdifferent diameters are necessary. The ends of these three cutters ofdifferent diameters will interfit as shown in FIG. 2 and not interferewith each other even though one cuts substantially to the central axisof the bit and all cut on the same conical surface.

FIGS. 3A, 4A and 5A illustrate the tooth structures of FIGS. 1 to 5somewhat more in detail. It will be understood that these toothstructures will be selected according to the known tooth structures mostsuitable for the particular formation being drilled. It is well knownthat in drilling soft formations of a certain character, chisel-shapedsharp crested teeth are employed, while in certain formations not quiteso soft more of a crushing action is required. FIGS. 3A, 4A and 5Aillustrate teeth which are somewhat blunt.

In FIGS. 7, 8, 9, 7A, 8A and 9A there is shown still another form ofcutter having teeth of the character known as insert or compact teeth.The bit body and bearing shafts for these cutters may be the same asthose illustrated in FIG. 1 and no additional description thereof willbe given at this point. However, the cutters are somewhat different. Onthe left-hand cutter body 42 illustrated in FIGS. 7 and 8 there is ablunt end 45 similar to the blunt end 26 of FIG. 1. However, the teethon the sides or conical surfaces of this cutter are not formed from themother metal of the cutter body and then hardened or hard surfaced as isthe case in FIG. 1, but instead are formed by embedding inserts of hardmaterial such as tungsten carbide or the like as illustrated at 46 and47. The inserts 47 cut substantially at the outer edge of the bottom ofthe hole, and the inserts 48 which are provided in the shank of the bitbody serve to assist the inserts 47 in maintaining the gauge of thehole.

In the blunt end 45 of this body 42 there are provided a number of teethformed of such inserts, those at 49 being located around the margin ofthe blunt end and a single such insert at 50 being located centrallywith respect to the axis of rotation of the cutter.

In similar fashion, the cutter body 52, of which only the blunt end andnose portion is illustrated, has cutting teeth inserts 53 located on itsconical sides, with one circularly arranged group of insert teeth 54 andone central insert tooth 55 in the blunt end or nose portion of thecutter.

The cutter bod-y 56, of which likewise only the nose portion isillustrated, has teeth formed of inserts 57 located in the conical sidesthereof and a symmetrically arranged group of three inserts formingteeth 58 located in the blunt end thereof.

In the case of this bit arrangement the cutter 56 has the smallest bluntend, the cutter 42 next, and the cutter 52 the largest blunt end. Ineach case the action of cutting the core will be similar to thatdescribed in connection with FIGS. 1 to 5, inclusive, except that withthis form of cutter the cutting is done by more impact and pressure thanby chipping and actually cutting. However, the scraping movement of eachcutting tooth across the sloping upper surface of the core will in everycase be similar to that described in connection with FIGS. 1 and 2.

FIGS. 7, 8 and 9 each show two only of the cutters of a three-cutterbit, each figure being along the plane of the axes of the two cutters,in order to show how each cutter interfits, and does not interfere, withthe other two cutters.

With reference to FIG. 6 there is illustrated a still furthermodification which willbe found highly advantageous in connection withcertain types of formation. The blunt end 60 on the cutter of which onlythe nose portion is shown in this figure has sharp crested teeth such asabove described, but insteadof being arranged in radially disposedpositions as in the previously described cutters, these are skewed, and,in the form shown, are arranged in spiral rows, two spiral rows beingshown in this figure. The outer teeth 61 of these spiral rows arepreferably somewhat longer along their crests than the inner teeth 62-of these rows. However, in each instance the crest of the tooth extendsalong the length of the spiral row. The effect of this arrangement isthat when this cutter nose rotates the cutting crest will move acrossthe formation of the core with a motion which is partly transverse tothe cutting crest but which also has a component in a directionlongitudinally of the cutting crest so as to produce a shaving orshearing action with a slight sawing effect. The efiiciency of such amovement in cutting is well known and this novel arrangement of cuttingteeth tends to produce the desired efiect.

While described and illustrated as applied to a threecutter bit, it willbe apparent that this invention is applicable to bits with any number ofcutters. Also, the teeth or cutting elements can be of any conventionalmaterial such as diamonds, tungsten carbide, or other substance or anycombination thereof.

It will also be apparent that in each form illustrated the cuttingaction of the inner end teeth will produce a thrust reaction on thecutter which will be directly opposed to the thrust reaction on thecutter resulting from the cutting action of the gauge forming teeth.These will thus tend to balance each other and reduce the thrust bearingcapacity required for the cutter.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forth,together with other advantages which are obvious and which are inherentto the apparatus.

Itwill be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed 1s:

1. In a drill bit comprising a hollow body having a connection for adrill stem at its upper end and a cutter carrying leg extendingdownwardly from aipredetermined position on its circumference, adownwardly and in wardly extending cutter bearing on said leg adjacentits lower end, and a generally frusto-conical cutter, with cutter teethon its outer conical surface, journaled on saidbearing and disposed withits larger end radially outermost from the bit axis, the improvementwhich comprises said cutter terminating at its inner extremity short ofthe bit axis and cutting teeth on said extremity directed axially of thecutter with cutting extremities separated from each other by reliefareas, the cutting extremities having an aggregate area exposed axiallyof the cutter less than the aggregate area separatingthe cuttingextremities which cutting extremities in rotation define a surface whoseelements form a greater angle with the cutter axis than the cutter axisdoes with the bit axis for engaging the corewhich would be left by theother teeth of said cutters at the center of a bore being drilled bysaid bit.

2. A drill bit improvement as set forth in claim 1 in 81 which at leastpart of thecutterteethon said cutter inner extremity have elongatedcrestsdisposecl at an angle to the direction of motion of the teethacross a coreformation to cut the same with a mot-ion having at alltimes sidewise and endwise components of the teeth across the formationand inwardly extending cutter bearing on each of said legs adjacent itslower end, and a generally frusto-conical cutter with cutter teeth onits outer conical surface journaled on each bearing and disposed withits larger end radially outermost from the bit axis, the improvementwhich comprises each cutter terminating at its inner extremity short ofthe bit axis and cutting teeth on each such inner extremity directedaxially of the cutter and having cutting extremities which in rotationdefine a surface whose elements form a greater angle with the cutteraxis than the cutter axis does with the bit axis for engaging the corewhich would be left by the other teeth of said cutters at the center ofa bore being drilled by said bit, the effective cutting extremities ofcorresponding ones of said axially directed teeth of different cuttersbeing located equidistant from the rotational axis of the bit measuredparallel to the cutter axis, whereby the cutting teeth of all cutterswill in rotation operate on the same surface of the formation, and theend cutting teeth on certain of said cutters being of less radial extentwith respect to their cutter axis than are the end cut-ting teeth onother of said cutters, whereby interference between the end cuttingteeth on adjacent cutters will be avoided.

5. In a drill bit comprising a hollow body having a connection for adrill stem at its upper end and a cutter carrying leg extendingdownwardly from a predetermined position on its circumference, adownwardly and inwardly extending cutter bearing on said leg adjacentits lower end, and a generally frusto-conical cutter, with cutter teethon its outer conical surface, journaled on said bearing and disposedwith its larger end radially outermost from the bit axis, theimprovement which comprises said cutter terminating at its innerextremity short of the bit axis and cutting teeth on said extremitydirected axially of the cutter and having cutting extremities which inrotation define a surface whose elements form a greater angle with thecutter axis than the cutter axis does with the bit axis for engaging thecore which would be left by the other teeth of said cutters at thecenter of a bore being drilled by said bit, said axially directed teethbeing arranged with at leastone cutting tooth interrupting everydiametrical pat-hacross the inner extremity of such cutter so that atall times at least one tooth will be in cutting engagement with suchcore.

References Cited in the file of this patent UNITED STATES PATENTS1,143,271 Hughes June 15, 1915 1,306,674 Esseling June 10, 19191,480,014. Scott Jan. 8, 1924 1,647,753 Scott et al Nov. 1, 19271,754,848 B-ull Apr. 15, 1930 1,909,128 Scott et al May 16, 19332,038,386 Scott et al Apr. 21, 1936 2,038,387 Scott Apr. 21, 19362,177,332 Reed Oct. 24, 1939 2,381,415 Williams Aug. 7, 1945 2,587,429Arutunoff Feb. 26, 1952 2,774,571 Morlan Dec. 18, 1956

1. IN A DRILL BIT COMPRISING A HOLLOW BODY HAVING A CONNECTION FOR ADRILL STEM AT ITS UPPER END AND A CUTTER CARRYING LEG EXTENDINGDOWNWARDLY FROM A PREDETERMINED POSITION ON ITS CIRCUMFERENCE, ADOWNWARDLY AND INWARDLY EXTENDING CUTTER BEARING ON SAID LEG ADJACENTITS LOWER END, AND A GENERALLY FRUSTO-CONICAL CUTTER, WITH CUTTER TEETHON ITS OUTER CONICAL SURFACE, JOURNALED ON SAID BEARING AND DISPOSEDWITH ITS LARGER END RADIALLY OUTERMOST FROM THE BIT AXIS, THEIMPROVEMENT WHICH COMPRISES SAID CUTTER TERMINATING AT ITS INNEREXTREMITY SHORT OF THE BIT AXIS AND CUTTING TEETH ON SAID EXTREMITYDIRECTED AXIALLY OF THE CUTTER WITH CUTTING EXTREMITIES SEPARATED FROMEACH OTHER BY RELIEF AREAS, THE CUTTING EXTREMITIES HAVING AN AGGREGATEAREA EXPOSED AXIALLY OF THE CUTTER LESS THAN THE AGGREGATE AREASEPARATING THE CUTTING EXTREMITIES WHICH CUTTING EXTREMITIES IN ROTATIONDEFINE A SURFACE WHOSE ELEMENTS FORM A GREATER ANGLE WITH THE CUTTERAXIS THAN THE CUTTER AXIS DOES WITH THE